Swapping HDD with OS between machines

I am replacing/updating my PC that I use in the office and taking the older one home to use. Both are Dell systems and will run WinXP Pro SP2. The new one is spec'd out with a 500GB HDD and the old one runs off a 120GB HDD. I run all my data off of an external HDD and I sync/backup the data from the external to the 120GB and that is only 11GB of data. I don't need all that HDD space on the office PC, I'd rather have the space on the old PC that I will take home to use. Is it possible, technically and legally to just swap the HDD's from the PC's and just load the appropriate drivers that are specific to the system config?

I can always just swap the HDD's and reformat/reinstall the OS's using the XP Pro software that is licensed to each system, but I don't want to spend that much time with the 2 OS reinstalls if I can avoid it and still have a stable system. The timing of the purchase was off and I didn't order it before the Xmas holiday when I was off and could do the laborious OS installs.

Thanks,
Jerold